1. Field of the Invention
The present invention relates to a postprocessing apparatus such as a printer and an MFP, and more particularly to a paper folding apparatus that rules a line on each of sheets and folds the sheets at a preprocessing stage for forming a saddle-stitched booklet using the sheets after image forming, and the invention also relates to a postprocessing apparatus using the paper folding apparatus.
2. Description of the Related Art
Conventionally, as a method for folding a batch of sheets into a booklet form after saddle stitching by a postprocessing apparatus, there is known a technique in which a binding position is pressed by a chopper, the batch of sheets is pushed into a nip of rollers, and a crease is formed by a pressing force of the rollers. According to this technique, however, since the booklet swells, the appearance and ease of use are inferior and many booklets can not be loaded. There is also a problem that in the case of cardboards or paper sheets that curl outward, even if crease is formed, the sheets can not be formed into the booklet form and open.
To suppress the swelling of the booklet, there is proposed a paper folding apparatus that previously rules a line at a predetermined position of a sheet in a preprocessing of booklet folding operation.
For example, Japanese Patent Application Laid-open No. 2000-272823 describes a paper folding apparatus that rules one straight line on a paper sheet while sandwiching the sheet between a first ruling roller having a groove of V-shaped cross section and a second ruling roller having an angular projecting central portion and carrying the sheet and then, folds the sheet.
Japanese Patent No. 2962862 describes a stitching/folding apparatus provided with a paper folding apparatus having a line-pressing portion having a line-pressing blade for ruling a line on a sheet-like work and a receiving blade placed below the sheet-like works for receiving a pressing force of the line-pressing blade. In the apparatus, the sheet-like works are continuously supplied one-sheet by one-sheet to the line-pressing portion so that one line is ruled on the sheet-like work and then the sheet-like work is fold.
In any of the paper folding apparatuses described in above publication and patent, since the number of line is one, if the number of sheets of paper of the batch is increased, even if the plurality of sheets are positioned for making a booklet, the positions of the lines formed on the sheets are misaligned. Thus, there is a problem that a ruling effect can not be obtained so much when the sheets are bound and folded.
On the other hand, a paper folding apparatus that always rules a plurality of lines has a problem that even when the number of a batch of sheets is as small as one to three, a plurality of creases are formed in a booklet, and the appearance of the booklet is inferior.